A microwave mixer is generally, a three port non-linear microwave device that takes an incoming low level radio frequency (RF) signal and modulates or mixes it with a strong signal from a local oscillator (LO) to produce signal frequencies including the sum, difference (IF) and cross-products of the RF and LO signals. Microwave mixers, therefore, are employed in devices where it is desirable to convert a higher frequency signal to a lower frequency signal including any receiver systems such as: Satellite Communication Receivers; Direct Broadcasting Satellite receivers; Up and Down converters and EW warfare systems; and so forth. The higher frequency signals are converted to lower frequency signals by providing optimum impedances for signal extraction. Microwave mixers are also employed in devices requiring upconverting a low frequency signal to a higher frequency signal. Microwave mixers are incorporated in a plurality of structural schemes including waveguide, microstrip, or coplanar waveguide, depending upon the application in which the mixer is desired to be used.
Early microwave mixers generally consisted of a point-contact diode fabricated by a metal whisker which forms a rectifying junction by contacting the surface of the semiconductor. These mixers, however, were generally insensitive to conversion loss improvements made by correct impedance matching as a result of their high series resistance, and thus significant gains in noise reduction were not realized.
The advent of Schottky barrier diodes, which are fabricated by plating, evaporating, or sputtering a variety of metals on n or p type semiconductor materials to form a rectifying metal-semiconductor junction, have resulted in mixers with lower conversion losses and noise because of their much smaller series resistances. Hence, microwave mixers have evolved into complex circuits that use two, four, or eight Schottky barrier diodes, single and dual gate field effect transistors (FETs) in microwave integrated circuit (MIC) or monolithic microwave integrated circuit (MMIC) technology.
Monolithic microwave integrated circuits (MMIC) are devices wherein all the active and passive circuit elements and associated interconnections of the device are formed either in sire on or within a semi-insulating semiconductor substrate by one or more well known deposition processes. Among the active circuit elements which can be formed on a semiconductor substrate are diode modulators, transistors, and switches.
The double-balanced mixer (DBM), using a quad diode ring (diode ring modulator), is frequently used in the industry. The term "double-balanced" is used to describe the fact that such types of mixers are capable of isolating both the RF signal and the LO signal voltages from the IF signal output. During one-half of the LO cycle, half of the diodes are in a high resistance state and half in a low resistance state. During the remaining half of the LO cycle, the diodes are in the opposite state. Hence, the mixer acts like a symmetrical switch, turning on and off at the LO frequency. This creams a signal at the output that has an average voltage of 0. DBM employs baluns which function to split the input RF and LO signals into two equal amplitude and phase reversed signals which are then applied to the opposite nodes of the quad diode ring. The large signal LO drive mixes with the low level RF signal to generate the required IF signal along with various other cross products.
Generally, prior art broadband DBMs comprise nonplanar or special baluns which are not compatible with monolithic microstrip integrated circuits. It is, however, desirable to implement DBMs using MMIC techniques for a number of reasons. These reasons include the fact that MMIC's are fabricated through batch processing which results in potentially low-cost circuits; MMIC's have improved reliability and reproducibility through minimization of wire bonds and discrete components; their small size and weight advances the art of miniaturization; and MMIC's offer circuit design flexibility and multi-function performance on a chip.
The DBM circuit of the present invention allows the employment of standard well known microstrip technology to effectuate a broadband MMIC DBM.
It is well known in the art that conversion loss is an important figure-of-merit with respect to microwave mixers. Conversion loss is defined as the difference between IF output power and RF input power to the mixer. In other words, it is the measure of loss encountered when the RF input signal gets scaled in the frequency or "downconverted". Broadband systems demand linear input vs. output characteristics thus, the mixer must display a good conversion loss flatness over a broad frequency band. Up until now, prior art DBMs implemented as microstrip MMIC have displayed poor conversion loss flatness over a broad frequency band.
It is, therefore, an object of the present invention to provide a double-balanced microstrip mixer circuit configured for use in a monolithic microwave integrated circuit which enables broadband frequencies to be applied to the diode ring modulator independent of the impedances of the baluns and which achieves a flat conversion loss over a broad frequency band.